1. Field of Invention
The present invention relates generally to equipment used in semiconductor processing. More particularly, the present invention relates to a thermophoretic wand which enables front and back surfaces of an object to be protected from particles while being transported.
2. Description of the Related Art
In extreme ultraviolet (EUV) lithography, as well in other lithography processes, clean, fragile items such as wafers or reticles must be handled such that critical surfaces of the items remain relatively uncontaminated by particles and are not subjected to factors, e.g., bending moments or excessive heat, which may adversely affect the integrity of the items. In the case of reticles used in EUV lithography, pellicles are generally not suitable for use in protecting patterned surfaces of the reticles, as will be appreciated by those skilled in the art. Various tools, as for example thermophoretic vacuum wands, have been developed for use in handling items such as wafers and reticles to reduce the likelihood that items such as EUV reticles may become contaminated with particles or have their integrity otherwise compromised.
Thermophoresis refers to a force which occurs in a temperature gradient, and causes particles in the temperature gradient to move from a hotter region to a colder region. That is, thermophoresis involves the attraction of particles to colder areas. Hence, a thermophoretic vacuum wand is generally heated, and therefore warms an object that is to be carried by the wand relative to the environment surrounding the object to create a thermophoretic force which often repels airborne particles from the surface of the object.
FIG. 1a is a diagrammatic representation of a thermophoretic wand or holder which is arranged to carry an object such that top or front surface of the object, i.e., the surface of the object that is exposed while carried on the wand, is substantially protected from particle contaminants. A thermophoretic wand 100 generally includes a head area 104 which is arranged to support an object (not shown). Head area 104 includes a plurality of openings 106 through which a vacuum may be applied. The vacuum may be provided through a line 116 that is controlled or otherwise regulated by a valve 120. The application of a vacuum through openings 106 allows an object (not shown) to be held against head area 104.
Wand 100 includes an electric heater which is powered by electrical line 112, and is arranged to heat the surface of head 104. FIG. 1b is a diagrammatic cross-sectional side view representation of a head of a wand, i.e., head 104 of wand 100 of FIG. 1a, which includes an electric heater. By heating the surface of head 104, an object 138 that is ultimately clamped to head 104 is also heated, typically to a temperature that is higher than an ambient temperature around wand 100. Hence, thermophoretic forces protect a top surface 142 of object 138 that is exposed to the environment from particles by causing any particles near top surface 142 to effectively be attracted away from exposed top surface 138 to the cooler surrounding environment.
Head 104 includes an electric heater 132 which is arranged to heat a platen 126 and, hence, a top surface 117 of platen 126, to a temperature that is higher than a surrounding environmental temperature to create thermophoretic forces to drive particles away from top surface 142 of object 138 when object 138 is effectively suctioned against top surface 117 of platen 126.
While heating object 138 is generally effective in substantially repelling particles from top surface 142, top surface 117 is often warmer than a back surface 139 of object 138, particularly prior to contact being made between top surface 117 and back surface 139. When top surface 117 is warmer than back surface 139, and top surface 117 is in proximity to back surface 139, as for example just prior to object 138 being clamped against top surface 117, thermophoretic forces may drive any particles positioned between back surface 139 and top surface 117 towards back surface 139, as back surface 139 is cooler than top surface 117.
In this description and below, a wand is described as contacting and clamping the bottom surface of an object. However, it should be appreciated that a wand may also clamp an object from the top. In such a case, the surfaces described as “top” may be interchanged with “bottom” or “back” surfaces.
With reference to FIGS. 2a and 2b, the attraction of particles to the back surface of an object to be carried on a thermophoretic wand will be described. FIG. 2a is a diagrammatic cross-sectional side view representation of a head of a thermophoretic wand, an object to be carried on the head, and particles near a contact surface of the object. A head 204 of a thermophoretic wand includes a platen 226 that is heated by an electric heater 232 or a heating element. As previously mentioned, by heating platen 226 to a temperature that is higher than that of an environment surrounding the wand, when a back surface 239 of an object 238 is clamped against a top surface 217 of platen 226, object 238 may be heated such that any particles (not shown) near a front or top surface 242 of object 238 are repelled from top surface 242. The onset of a thermophoretic force that effectively protects top surface 242 from particles (not shown) may be delayed if the finite heat capacity of object 242 is such that the temperature rise of object 242 from an initial temperature to a heated temperature is relatively slow. Any delay in the onset of a thermophoretic force may cause some particles (not shown) to come into contact with top surface 242, or may delay usage of the wand.
While particles (not shown) positioned above top surface 242 of object 238 may be repelled away from object 238 due to thermophoretic forces once object 238 is heated, particles 250 which are located between back surface 239 of object 238 and top surface 217 of platen 226 are often attracted to back surface 239. This attraction may arise when electric heater 232 heats platen 226 to a temperature that is higher than the temperature of object 238, and thermophoretic forces cause particles 250 to be attracted to back surface 239. When such an attraction occurs, particles 250 may be “stuck” to back surface 239, and remain stuck to back surface 239 even after object 238 is heated by head 204 to create thermophoretic forces to repel particles (not shown) away from top surface 242 of object 238, as shown in FIG. 2b. 
Particles 250 may remain substantially stuck to back surface 239 of object 238 when object 238 is removed from head 204. When particles 250 remain stuck to back surface 239, and object 238 is used in a process such as an EUV lithography process, the integrity of the process may be compromised. For example, when object 238 is a reticle used in an EUV lithography process and particles 250 are essentially sandwiched between back surface 239 and a chuck on which object 238 is positioned, distortions may be caused in an image projected off of top surface 242, which may be a patterned surface. Alternatively, when object 238 is a wafer, the integrity of object 238 may be compromised if particles 250 affect the accurate positioning of object 238 in a wafer chuck.
In addition to particles 250 adversely affecting processes involving object 238, heating object 238 using head 204 may not be desirable, as raising the temperature of object 238 may cause object 238 to expand or become distorted. Distortions may include thermal distortions on a patterned surface on object 238 when object 238 is a reticle.
Therefore, what is needed is a system and a method which is suitable for use as a holder of a clean, fragile object that does not heat the object and minimizes particle contamination on both top and back surfaces of the object. That is, what is desired is a wand apparatus which protects front and back surfaces of a clean, fragile object without the application of heat to the object.